Dense patterns are understood to mean patterns, some elements of which are very narrow and separated by intervals that are very narrow. With photolithography, a sensitive layer is exposed, for example through a mask, for example to an ultraviolet beam, typically to a wavelength of 193 nanometers. The density of the patterns, in other words the resolution that can be achieved in the etching of the pattern, is linked to several parameters but it firstly depends on the wavelength used; the shorter it is the higher the density. The density of the patterns can be increased by using extreme ultraviolet (notably at 13.5 nanometers); it can also be increased by using an exposure to an electron beam of very small diameter, but the process of etching is much slower because the writing of a pattern is carried out point by point and not through a global mask defining the patterns; furthermore, narrow electron beams can generate phenomena of dispersion and back-scattering of electrons to the detriment of the resolution.
In the field of memories, in order to increase the possible density of the lines needed in order to fabricate these memories, a known solution is to carry out the following steps:                formation of a sacrificial layer on a substrate,        etching of the sacrificial layer according to first strips,        formation of spacers along all the edges of the sacrificial layer thus etched;        removal of the sacrificial layer so as to leave only the spacers remaining forming the dense lines sought.        
The invention provides an improvement of this type of method, suited to the fabrication of superposed networks of conductors, separated by an insulating layer and connected by conducting vias through this insulating layer.
A problem is then posed which is the alignment of the vias with respect to at least one of the two conducting networks and the invention provides a solution using common steps for the formation of the second conducting network and of the vias, and, for the vias, using an alignment by spacers which are used to define conductors—, in such a manner that the vias are perfectly aligned with the second network even when the latter is very dense.